Cakewalk UA-25 Ex Round-Trip Latency with Mac Pro

I'm using UA-25 Ex with Quad-Core i7 config Mac Pro Mac OSx Lion

It runs stable @ 24-bit 48 Khz  I/O Buffer Size 64 Samples, Output Latency 3.4 ms & Round-Trip Latency 12.6 ms. These are Logic Audio Preferences. 

In System Preferences, I tried to use Audio Buffer size of UA 25 Ex @ Minimum, but it caused unexpected loads on CPU. Runned unstable. I raised the Audio Buffer size 2 strips more from minimum. And it runs stable.  

With these config : Round-Trip Latency is 12.6 ms. I wonder, am I able to use my UA 25 EX's Hardware Direct Monitoring specification in Logic ? If I use Di-Monitoring, does 12.6 ms mean anything for me ? For Both Midi and live instrument recording ?